A known scanner includes an automatic document feeder (hereinafter, referred to as the “ADF”). The scanner is configured to read a document using a reader fixed at a predetermined reading position, while feeding the document using the ADF. The scanner includes a transparent platen between the reader configured to read documents and a document fed by the ADF. Accordingly, the reader reads a document, via the platen. When dust adheres to the platen, the reader continuous reads the dust on the platen while the reader reads the document fed by the ADF. Therefore, an output image has a black streak extending in a document feeding direction.
To prevent or reduce such problem, the known scanner is configured to detect dust adhering to the platen before the scanner starts reading a document. The reader, e.g., a line sensor, is placed at a position avoiding a dust detected position.